Predation risk from piscivorous fish influencing the diel use of macrophytes by planktivorous fish in experimental ponds

This study investigated interactions between zooplanktivores (roach and perch) and piscivores (pike and large perch) in experimental ponds (16 m(2)) with open water habitat and three densities of natural macrophytes. Fish habitat selection was determined both day and night and was supported by daytime observations to study anti-predator behavioural patterns. Diel migration out from among macrophytes was seen in the absence of predators, particularly for roach, which changed from 13% of individuals being in open water during the day to 90% at night. The risk of predation from piscivores influenced the habitat selection of the zooplanktivores. Roach seemed to be the most vulnerable to predation from pike and selected the open water (90-92%) during daylight hours, but kept a 1-m distance from the macrophytes edge. The presence of pike thus reduced the use of macrophytes by roach, which in turn may improve macrophytes and the edge area as a refuge for zooplankton. Pike appeared to have less impact on the gross habitat selection of 0+ perch, which were associated with the macrophytes (58-89%), though they were still vulnerable to predation. Adult perch, which was a generally less effective predator than pike, showed subdued behavior, concealing themselves in the macrophytes most of the time. No effect of the simultaneous presence of two predator species was found.